Method of cleaning sulfide contaminated condensates

ABSTRACT

A method to remove sulfides and other volatile contaminants from liquor vapor condensate in a pulp manufacturing process, where the mentioned liquor vapor condensate is fed into a stripper, which is part of a closed loop system including the stripper, a regenerative thermal oxidization process (RTO) and a SO 2  scrubber, in which loop a gas is circulated, preferably air, and such components formed or stripped off, in this loop whereafter the circulating gas is stripped off sulfides and other volatile components from the liquor vapor condensate, whereafter the gas stream is fed into a RTO process, where the stripped off contaminants are combusted are under formation of SO 2  and thereafter the SO 2  enriched gas is led to a SO 2  scrubber, where preferably alkali is used as absorption medium, and thereafter the circulating gas is returned back into the stripper.

In producing chemical pulp according to the Kraft chemical pulp process,waste liquor is produced that is being evaporated prior to burning.During the evaporation process, liquor vapor is stripped off, which inaddition to water vapor, also contains certain volatile contaminants.Such contaminants are hydrogen sulfide, methylmercaptan,dimethylsulfide, methanol, terpenes etc. At the evaporation which takesplace as a so called multiple effect evaporation with a number ofstages, effects (normally 4-7), the liquor vapor is also condensed inmultiple stages, whereby also large amounts of the volatile contaminantswill condense. The condensation takes place in at least as many stagesthere are effects. This means that the quality of the condensate variessignificantly from the different stages of the evaporation. Normally 2-3different condensate qualities are being separated, where each one is amixture of condensates from a number of effects. The dirtiestcondensate, (foul condensate), is normally treated in a steam stripperwhere the volatile components are flashed off. This foul condensate istypically a small amount of the total condensate flow and therefore thesteam economy is not affected to any higher degree of the fact thatsteam is used as the stripper gas. The investment cost can also be keptat a minimum.

The purity of the other condensate qualities is highly dependent on theamount of foul condensate. If the amount of foul condensate is increasedthe contaminated condensates will be cleaner. A too high amount of foulcondensate however the operating and investment cost for the steamstripper system will increase.

The other, less contaminated condensates can to a limited extent be usedas process water in dependency of their cleanliness. However if thecondensate is too contaminated it can not be re-used but must instead bedischarged to the recipient subsequent to some form of treatment

The primary limiting factor for the use of the contaminated condensateas process water is the content of sulfides, as these can give anunpleasant small and taste to the pulp. It also creates a significantproblem for the working environment. Also terpenes give a smell. Theterpenes however are normally present at very low amounts in the lesscontaminated condensates.

The technology available to clean these condensates is predominatelysteam stripping. Since the various condensate flows are very large, thesize of the stripper will be significant and a large amount of steamwill be required for stripping. The steam volumes will be so large thatit will definitely not be economical to use fresh steam. On the otherhand it is possible to use flash steam driven off from the evaporationof the waste liquor, in multiple effect evaporation for the stripping.The steam leaving the stripper then can be regained as heat in the nextevaporation effect. The cleaning efficiency of such a stripper ishowever limited since the flash steam from the preceding effect isalready contaminated with sulfides, which limits the degree of purity ofthe output condensate. Primarily the cleanliness is limited regardingsulphides, as the waste liquor can have a considerable content ofsulphides. This sulphide content is dependent on that steam is normallytaken from the first effect, where the temperature is rather high, whichgives an increased sulphide content.

Another drawback is that when the steam passes through the stripper, itloses pressure and volatile components are enriched. These two thingswill reduce the condensation temperature, which means that thetemperature difference available at the evaporation is reduced. Theenergy and capital cost are both negatively impacted thereby.Furthermore the evaporation plant and the stripper are completelyintegrated, whereby these two parts can not be independently operated.

The dimensions of the stripper also will become large, which meanssignificant costs for the equipment.

In a conventional steam stripper also other volatile components, such asmethanol, are stripped off.

Air can be used to in lieu of steam to strip the condensates. A bigdrawback with this method is that air is being contaminated and must becleaned in some way. The air volumes can also be very large.Additionally the condensate is being cooled down by the air, which has alower wet bulb temperature as compared to the temperature of thecondensate. For these reasons pure air stripping is not a realisticalternative for a modern and environmentally friendly pulp mill.

The present invention provides a possibility to strip off primarilysulfides at a very high efficiency from liquor-steam condensates from apulp manufacturing process, and simultaneously to take care of thesulphur, thus that it will not contaminate the environment. This isbeing done in a closed loop concept that is comprised of three processsteps, where the sulfides are stripped off from the condensate, thestripped off sulfides are being oxidized to sulphur dioxide, and toabsorb the sulphur dioxide formed.

The three process steps are consequently:

1. Stripping off sulphides from liquor-steam condensate

2. Oxidation of combustible components such as sulphides and hydrocarbons.

3. Absorption of sulphur dioxide.

By integrating these three process steps (1, 2, and 3) in a closed loopcycle, the cleaning of condensates can be done with a high efficiency,good heat economy, and minimal impact on the environment

BRIEF DESCRIPTION OF THE DRAWING

The drawing schematically shows the various process steps in accordancewith the invention.

DETAILED DESCRIPTION

The invention will in the following text be exemplified with referenceto a scheme show in the attached drawing.

In the present invention a gas is used as a medium for stripping off thesulphides from the condensate. This gas is substantially and preferablycomposed of air. This process step is normally designed as a scrubbercolumn 1, where the gas 4 is introduced in the lower section and thecondensate 5 in the upper section, thus that the gas and the condensatemeet in counterflow contact. The contact means in the scrubber can betrays or packing material. The gas 6 leaving the scrubber will containsulphides in form i.a of hydrogen sulphide and methyl mercaptan, butalso organic compounds such as methanol and terpenes. This contaminatedgas 6 is led to an oxidization process 2, where the gas is treatedcounterflow in a regenerative beat exchanger. The gas 7 from theoxidization step contains partly sulphur dioxide These gases are thenfed to a contact device, in form of a SO₂ scrubber 3, where the sulphurdioxide is absorbed in a preferably alkaline solution 8. The gas is thenreturned to the condensate scrubber to be used again as a strippingmedium. In this manner is formed a closed the loop. Since oxidation inthe closed loop consumes oxygen is necessary to add fresh oxygen.Additional oxygen can be added by supply 9 preferably of air or someother oxygen containing gas. The system does not allow for gasaccumulation in the loop and therefore a minor portion of the gas 10must be bled off. The gas circulation through the three process steps isaccomplished by the use preferably of a fan.

Since the gas in the closed loop is primarily being circulated, anelevated level of various gas components can accumulate to rather highlevels. However, since only a minor portion of the gas is bled off, thedischarge of components harmful to the environment, will be limited, inspite of high concentrations in the system.

A method of improving the cleaning of the condensate in the stripper isto increase the level of SO₂ after the SO₂ scrubber (3). Such a methodwill result in that the condensate in the stripper (1) will get a lowerpH value. A lower pH value in turn gives a better stripping of sulphidesand makes possible an almost complete stripping of sulphides. This wouldotherwise be difficult to achieve since the condensate contains asmaller amount of alkali components, i.e. ammonia, which would increasethe pH value of the condensate when the acidic sulfides are strippedoff. An alkali component such as ammonia will remain in the condensateat a lowered pH. Thereby is avoided discharge of ammonia, which shouldotherwise be transformed to Nox, after the oxidation process.

An increase of the SO₂ concentration after the SO₂ scrubber (3) can beobtained by adjusting the supply of alkali to this stage thus that thete absorption medium will get a comparatively lower pH. The lower the pHthe higher the SO₂ concentration in the gas leaving the scrubber (3).The higher the SO₂-level in the gas, which constitutes the strippermedia, the better the efficiency of stripping off sulfides from thecondensate. In turn this effect can be utilized in such a way that theratio between the condensate flow and stripper gas flow can be increasedwith continuos good sulphide stripping. This in turn implies an elevatedlevel of sulphides in the stripper off gases, which in turn means anincreased SO₂ level after the oxidization step. In this way the SO₂level in the entire system can be significantly increased. This givesthe following benefits the SO₂ concentration after the SO₂ scrubber canbe:

1. Production of a sodiumbisulfite solution with a relative low pH ismade possible.

2. The size of the plant can be reduced

3. NO_(x) emission is reduced (see above)

The first benefit is accomplished since an increased SO₂ level in a gas,from an equilibrium point of view, gives a lower pH in the absorptionmedium. Since the addition of alkali is reduced a bisulfite solution isformed. This acid can be utilized as acidification in e.g. the bleachplant or the tall oil plant. An increased SO₂-level in the recirculatedgas results however in an increased SO₂ discharge from the system viathe bleed off to the atmosphere (10). Connecting a scrubber in thispoint, to absorb SO2 can cure this. A scrubber in this position ispreferably designed with multiple absorption steps, e.g. of the samedesign as the stripper. It could be so that only SO₂ is permitted to beabsorbed in this position. In that way the SO₂ scrubber (3) can beeliminated from the system.

The second benefit follows the fact that the circulating gas volumesubstantially determines the size of the equipment Since an increasedSO₂ content facilitates a higher ratio of condensate/stripper gas flow,the gas flow in the system can be reduced.

The cleaned condensate will contain very low levels of sulphides andalso any terpenes will be stripped off. This will give a condensatewhich is rather free from nasty-smelling contaminants. Methanol isanother significant contaminant in black liquor condensate.

Some of the methanol will be stripped off in the stripper and some willstay in the condensate. The amount stripped off methanol is dependent onthe ratio of supplied condensate to gas and the volume of the circulatedgas.

The heat economy in the system is excellent since no external heatenergy must be added. In the oxidation stage, heat is furthermoregenerated. This energy can compensate for various energy losses in thesystem, and any surplus can be absorbed as heat in the outgoingcondensate. In other systems, where for example air is used as strippergas, a significant amount of heat is absorbed in the air since the warmcondensate transfers water vapor in contact with air. This cools downthe condensate, which is avoided in the present invention, where anypossible evaporated water vapor is returned to the system. It might alsobe possible to recover heat from the system by implementing a heatexchanger in the system. With such a heat exchanger, which cools thesystem, the temperature can be controlled.

There might also be a need to supply heat to the system. One reasoncould be to avoid oversaturated gas in certain parts of the system. Asthe recirculated gas, for instance after the stripper, is saturated withwater vapor there is a risk that water droplets will fall out asmoisture in the gas. By heating the gas, it would be possible toeliminate that moisture.

The investment costs and the size of equipment is mainly directlyproportional to the amount of recirculated gas. For that reason it isimportant to minimize the gas recirculation. This will consequently havean impact on the methanol removal. It is therefore reasonable to countwith a certain amount of methanol still remaining in the condensate.Methanol, as a pollutant in the condensate can be a drawback if thecondensate is discharged to the recipient. If the condensate is beingrecirculated back into the process, e.g. as process water in the bleachplant, brown stock washing or limewashing, then the condensate isexcellent in spite of the methanol content.

Methanol has a positive impact on bleaching, it acts as a radicalscavenger and it also increases the solubility of lignin. Furthermore,this condensate is metal free. Normal process water prepared from nearbywater streams always contains a certain amount of metals, such as i.a.transition metals. These transition metals can be very harmful for thebleaching process since they decompose the bleaching agents such ashydrogen peroxide. Since the methanol act as a radical scavenger, thedegradation of cellulose molecules will decrease. A metal freecondensate used in the bleach plant therefore has significant benefitsin spite of a certain methanol content. By recirculating the condensateto the process a discharge of oxygen consuming matters is avoided. Themethanol enrichment in the process is very marginal, since the dischargeof methanol from the process is relatively large for each process cycle.

The stripping of condensate can be performed in several different ways.The type of equipment chosen shall be an equipment having a very highstripper efficiency. Such type of equipment ought to have severalequilibrium steps, where the condensate meets a counterflow of gas.Examples on such equipment are columns with trays or packing material.This is well defined in the technical literature, such as i.e. “Perry'sChemical Engineers' Handbook”, MacGraw-Hill Book Company, 1984.

The oxidization process can be done in different ways, but therelatively low concentrations of combustible components require certainprerequisites for this type of process. A relatively high temperature isneeded in order to oxidize the combustible components. A regenerativethermal oxidization process (RTO) is preferred where the gas is treatedin a heat exchanger under such temperature conditions that almost acomplete oxidization takes place. Example on such a process is describedin the patent application PCT/SE85/00257.

Scrubbing of the SO₂ gas can be done with an alkaline solution At a pulpmill there is a surplus of alkaline process fluids. One such fluid isoxidized white liquor. In the oxidized white liquor the sulfides havebeen removed by oxidization. White liquor is such a strong alkali thatSO₂ easily can be absorbed. One equilibrium stage is sufficient. Aventuri scrubber is a piece of equipment wherein one equilibrium stageis almost achieved. A relatively high gas velocity can be maintained ina venturi scrubber, which makes it compact. The scrubber medium iscirculated through the venturi.

The pH of the scrubber medium shall be controlled in order to controlthe SO₂ level in the gases leaving the scrubber. The venturi scrubberhas also a significant benefit in that the circulating liquid can have arelatively short residence time. This implies a fast control of the pHin the scrubber. As the scrubber has only almost one equilibrium stageinstead of several, a rapid response time is also achieved.

What is claimed is:
 1. A method of removing sulphides and other volatilecontaminants from liquor vapor condensate from a pulp manufacturingprocess, comprising, feeding said liquor vapor condensate from a pulpmanufacturing process into a stripper (1), which is part of a closedloop comprising said stripper (1) a regenerative thermal oxidizerprocess (RTO)(2) and an SO₂ scrubber (3), in which loop a gas (4),comprising air and components formed or stripped off in the loop, arecirculated, and where the circulating gas is used to strip off sulphidesand other volatile components from the liquor vapor condensate from apulp manufacturing process (5), whereafter the gas stream (6) exitingsaid stripper (1) is fed into said RTO-process (2), where the strippedoff components are combusted forming an SO₂ enriched gas, and thereafterfeeding the SO₂ enriched gas (7) to the SO₂ scrubber (3), whereafter thecirculating gas is returned to the stripper (1).
 2. A method as claimedin claim 1, wherein the SO₂ scrubber (3) is part of the closed loop. 3.A method as claimed in claim 1 or 2, further comprising bleeding offfrom said loop a portion of the gas (10) in said loop, at the same timeair or oxygen containing gas (9) is supplied to said loop.
 4. A methodas claimed in claim 3, wherein the gas (10) being bled off from thesystem is minimized by adding pure oxygen or an oxygen enriched airmixture to said loop, necessary as make up gas (9) for said oxidation.5. A method as claimed in claim 3, further comprising scrubbing saidbled off gas (10) with regard to SO₂ in a separate scrubber.
 6. A methodas claimed in claim 1, wherein alkali (8) is used as an absorptionmedium in said SO₂ scrubber.
 7. A method as claimed in claim 6, whereinaid absorption medium is oxidized white liquor.
 8. A method as claimedin claim 1 wherein the degree of acidification in the SO₂ scrubber (3)is controlled to ensure sufficient amount of SO₂ remaining in the gas(4) when it is returned to the stripper (1), where SO₂ acidifies thecondensate (5) and thereby contributes to enhance the stripping off ofsulphides from the condensate.
 9. A method as claimed in claim 1,wherein a heat exchanger is installed in the closed loop, to recover orsupply energy and thereby to control the temperature in the system. 10.A method as claimed in claim 1, wherein the amount of recirculated gasversus the amount of condensate is controlled for the purpose ofoptimizing the methanol content in the condensate.